1. Field of the Invention
The present invention relates to the field of bypass turbojet engines notably comprising an elongate bypass flow casing. It relates to any engine of this type, fixed to the fuselage of the aircraft or military engines.
2. Description of the Related Art
A bypass turbojet engine comprises a fan which when mounted at the front of the engine delivers a flow of compressed air which is divided into two concentric annular flows: a primary flow and a secondary or bypass flow surrounding the primary flow. The latter is guided toward the part of the engine that forms the gas generator and which comprises compression stages, a combustion chamber and a section comprising turbines by which the fan is driven. The primary flow containing the combustion gases is then ejected in an exhaust nozzle. The bypass flow is straightened downstream of the fan and is likewise ejected. In engines for civilian applications, it provides most of the thrust.
In one configuration, the bypass flow is guided in a fan duct which extends around the engine between the fan and the primary flow exhaust nozzle. It comprises two substantially cylindrical coaxial walls which between them delimit an annular space. The internal wall of the fan duct forms the shroud of the gas generator. The external wall of the fan duct forms a case which extends in the plane of the straightener vanes as far as the exhaust nozzle. The outer case of the fan duct is more commonly referred to as the outer fan duct or “OFD”.
The engine can be mounted under the wing of the aircraft or alternatively along the fuselage thereof, notably toward the rear. In that case, the engine comprises a fan duct as described hereinabove. The attachments of the engine to the aircraft are located in two transverse planes: one plane upstream passing through the structural front casing that is referred to as the intermediate casing and one plane downstream passing through the downstream structural casing that is referred to as the exhaust casing.
For securing the downstream end, in the case of an engine mounted on the fuselage, a structural ring is provided on the outer fan duct, OFD, this ring being connected by arms or connecting rods to the outer ring or shell ring of the exhaust casing. Patent application EP 2 022 973 in the name of the applicant company describes an example of an outer fan duct structure.
The connection between the two rings mentioned hereinabove may take the form of radial arms distributed all around the axis of the engine and secured rigidly to the two rings. The connection may also take the form of connecting rods which are inclined with respect to the axis of the engine. The connecting rods are secured to the two rings by attachments of the yoke and pin type. Such an attachment is formed of two single or double yokes, one of them secured to the end of the connecting rod and the other to the wall of the ring and through which a common pin passes. More particularly, the connecting rods are arranged in pairs, the connecting rods of each pair being tangential to the ring of the exhaust casing while at the same time converging onto an attachment of the ring of the outer fan duct.
Regardless as to whether the connection is formed of radial arms or of connecting rods, it is hyperstatic (statically redundant); loads thus pass through all the arms or connecting rods in a way that is determined by the relative strengths thereof. To simplify the description, both connecting rods and arms will be referred to hereinafter as connecting rods.
Because the path followed by tensile loads is the same as that followed by compressive loads, the connecting rods according to the prior art need to be sized to withstand both tensile loads and compressive loads.
In the solutions of the prior art, all the connecting elements: pins, yokes, connecting rods or arms, are sized to be able mechanically to withstand the loads resulting from the imbalance generated by the loss of a fan blade. This is in order to obviate the risk of the engine becoming detached in the event of such a critical situation arising. The mass of all the components that make up the connection is therefore high.
It is an object of the present invention to create a hyperstatic truss of connecting rods applied to the suspending of an engine casing ring inside another ring which, while reacting loads that might be generated by the breakage of a fan blade, makes it possible to reduce the overall mass in comparison with the solutions of the prior art.